As is well known, demands for a holographic digital data storage system that can store a large amount of data have been increasing. Therefore, various types of holographic digital data storage system have been recently developed for realizing high density storage capabilities.
The holographic digital data storage system allows a signal beam having information therein to interfere with a reference beam to generate an interference pattern therebetween and, then, controls the interference pattern to be stored in a storage medium made of an optical refractive crystal. The optical refractive crystal is a material which may react differently on different amplitudes and phases of the interference pattern.
Various holograms can be recorded in a same spatial location by changing an angle of incidence of the reference beam (angular multiplexing) and/or by moving the storage medium (holographic medium) to change a recording area (shift multiplexing), so that a great number of holograms of binary data can be stored in the storage medium.
Hereinafter, a conventional holographic digital data storage system, e.g., a holographic ROM system, using the angular multiplexing technique will be described with reference to FIGS. 1A and 1B (refer to “Holographic ROM system for high-speed replication”, ISOM/ODS 2002, pp. 144˜146).
As shown in FIG. 1A, the holographic ROM system includes a pick-up unit 100, a holographic medium 200, a motor 210, a control unit 300 and a signal processing unit 400. A plurality of data is recorded on the holographic medium 200 by using the angular multiplexing technique. The holographic medium 200 is rotated by the motor 210 operated under control of the control unit 300 during playback.
The pick-up unit 100 includes a case 101, a first actuator 102, a laser source 104, a PBS (polarization beam splitter) 106, an actuated mirror 108, an aperture 110, an objective lens 112, a second actuator 114 and a light receiving unit 116. Provided in the case 101 are the first actuator 102, the laser source 104, the PBS 106, the actuated mirror 108 and the light receiving unit 116.
During playback, the laser source 104 emits a laser beam with a constant wavelength, e.g., a wavelength of 532 nm. The laser beam of, e.g., only S type of linear polarization is provided to the PBS 106.
The PBS 106, which is manufactured by repeatedly depositing at least two kinds of materials, each having a different refractive index, serves to transmit one type of polarized laser beam, e.g., P-polarized beam, and reflect the other type of polarized laser beam, e.g., S-polarized beam. Therefore, the PBS 106 reflects the reference beam toward the actuated mirror 108.
The reflected reference beam undergoes another reflection at a predetermined angle by the actuated mirror 108, thereby being incident upon the holographic medium 200. In order to retrieve and reconstruct holographic data, the angle of incidence of the reference beam toward the holographic medium 200 should be identical to that of a reference beam employed during a recording operation.
When the reference beam reflected at the predetermined angle by the actuated mirror 108 is irradiated onto the holographic medium 200, the interference pattern recorded in the holographic medium 200 diffracts the reference beam to thereby create a reconstructing beam.
The reconstructing beam travels into the case 101 via the aperture 110 and the objective lens 112, and the objective lens 112 can be moved by the second actuator 114. Elements in the pick-up unit 100 can be moved by the first actuator 102. Furthermore, the case 101 is configured not to obstruct the traveling of the light.
After passing through the objective lens 112, the reconstructing beam is transmitted to the PBS 106 and then is reflected toward the light receiving unit 116 by the PBS 106.
The reconstructing beam received by the light receiving unit 116 is reproduced by the signal processing unit 400.
The holographic ROM system reproduces data, which have been overlappingly recorded at a first angle of incidence on the holographic medium 200 by using a reference beam having a phase conjugation with respect to the first angle, after which the actuated mirror 108 should be rotated in order to reproduce data, which have been overlappingly recorded at a second angle of incidence on the holographic medium 200 by using a reference beam having a phase conjugation with respect to the second angle.
Through the repetition of the above processes, the holographic ROM system reproduces the data recorded by the angular multiplexing technique.
However, if an angle of incidence of the reference beam toward the holographic medium 200 is changed, the incidence location thereof on the holographic medium 200 is changed and, accordingly, a radiating point of the reconstructing beam on the hologram medium 200 is also changed, as shown in FIG. 1B. Therefore, the reconstructing beam is deviated from an optical axis of the objective lens 112, so that a distortion of the reconstructing beam occurs thereby raising a problem of not being able to reproduce the data recorded on the holographic medium 200 exactly. As an extreme case, the reconstructing beam may be totally deviated from the objective lens 112, which may result in failure to reproduce the data recorded on the holographic medium 200.